But the fourth patient had the opposite experience. The person still showed persistent brain activity for up to 10 minutes after the final heartbeat, researchers wrote in a study published this week in the Canadian Journal of Neurological Sciences.

Patient No. 4's brain continued to fire off bursts of delta waves even after the person was declared dead, Loretta Norton and her colleagues of the University of Western Ontario found.

The researchers said they can't really explain what happened. Perhaps there was a human or equipment error that falsely simulated brain activity at the time of recording — though there's no sign that either a person or machine messed up.

"It is difficult to posit a physiological basis for this EEG [electroencephalographic] activity, given that it occurs after a prolonged loss of circulation," Norton and her team wrote.

The study is the latest effort by doctors to better understand what happens to our bodies after life support is withdrawn, which is an important question for organ donation.

Without a firm explanation, and given the tiny sample size — one patient — the doctors couldn't make any definitive conclusions about what their findings mean, except to say that more research is required.

"Further study of the [EEG] during the withdrawal of life-sustaining therapies will add clarity to medical, ethical and legal concerns for donation after circulatory determined death," they said.